The formation, ultrastructure and biochemistry of actin-containing filaments and filament bundles will be investigated in sea urchin coelomocytes. One class of coelomocytes, the leukocytes, undergoes a remarkable trasformation from a petaloid form to a filopodial form. The control of this transformation, involving a reorganization of filaments into bundles will be investigated with light and electron microscopy. The isolation of the filament bundles will allow analysis of their component parts and dissection of their function. Of particular interest are the control mechanisms present in this system which modulate the interaction of actin and myosin in producing the observed transformation.